Compact discs ("CDs") were originally designed as a storage/reproduction medium for the digital recording of music. Discs created for this purpose utilize a plastic-coated, aluminized surface having a standard diameter measuring 4.72 inches (12 cm.) and a standard thickness of 1.2 mm. Audio information is placed on only one surface of the disc (to simplify the manufacturing process) in digital form as a track of microscopic pits in the thin, reflective aluminum layer. It is the spacing and length of these pits, each being about 1 micron in length, which determines what data the disc player unit "sees".
In order to "play" such a disc, an infrared beam from a semiconductor laser is focused upon the pits through a series of lenses and prisms. Light striking a pit is scattered whereas light hitting the smooth surface between pits is reflected back to a prism through a photo-detector, i.e., a diode, that converts light into an electronic signal. The resultant on and off digital signal is then decoded and fed through an electronic filter, thus converting it from digital form into an analog signal suitable for any hi-fi amplifier.
More recently, audio compact disc technology of the type described above has been combined with a computer application in order to create an optical storage system known as the CD-ROM (compact disc, read only memory), utilizing either the standard 4.72 inch compact audio disc or the 51/4 inch size accepted by American manufacturers as a standard for digital information storage. Each CD-ROM can store over 500 megabytes of information, i.e., approximately 275,000 pages of text, which is 1,500 times that of a floppy disc and 10-50 times that of a hard disc.
Blank optical discs are constructed of a layer of a heat-sensitive metal film, an organic polymer (e.g., a plastic) or a combination of both, deposited upon an aluminum platter similar to that used for magnetic hard discs. Optical recording techniques deposit digital information upon the surface of the disc by marking this surface in a number of fashions. These techniques include: ablative recording, in which a laser creates a series of pits in the coating layer; bubble forming, in which the laser's heat generates a tiny volume of gas in the underlying polymer, which becomes trapped and thus forms a bubble in the surface of the metal film layer; and crystalline to amorphous phase change, in which the reactive layer of the disc is charged from light-reflecting to transparent by the writing laser.
The protective plastic coating on the surface of an audio CD or CD-ROM prevents dust or debris from reaching its reflective metal surface. In contrast, information stored on, for example, hard discs, must be carefully protected from dust, smoke, fingerprints, heat and other damaging conditions that don,t adversely affect a laser-read disc. Only significant scratches or blemishes can interfere with the laser light beam.
One common cause of such damaging scratches and/or blemishes is the removal and/or insertion of laser disc devices of the type described above from or into, respectively, a molded plastic storage case of the type commonly used to hold such discs. These cases contain many sharp edges which, if a disc is carelessly handled, can scratch or gouge the information-bearing surface of the disc and thus result in a loss of some or all of the information contained thereon.
A typical example of such a storage container is disclosed in U.S. Pat. No. 4,535,888 to Nusselder. This reference describes a plastic cassette suitable for the simultaneous storage of at least two discs having high information storage density such as, for example, an optically readable video disc or digital record. The cassette is provided with a box-shaped center portion having an intermediate bottom part respectively terminated at its upper and lower sides with a cover in the form of a box-shaped floor part. Each floor part is hingeably mounted at one side of the center part. The construction of this article enables a disc to be supported adjacent each floor portion and on either side of the intermediate bottom part by a raised seat and peg arrangement centrally disposed upon those parts. Storage cassettes constructed in this manner are commonly referred to as a "jewel-box" cassette due to their hinge-type construction.